Geores.org was originated from the idea to report about my research project in geophysics. The main focus is in technical realisation and the tests of devices and sensors for geosciences. I present research-journeys and research-projects in which I am or was involved. The goal of Geores.org is also to make these exciting geophysical developments accessible for everyone and produce the possibility that everybody can take part actively in the presented projects/researches.
Contents are in English and German!
The content of Geores.org is under Creative Commons-Lizenz.
N E W S
AGU Fall meeting 2011 in San Francisco
From the 5. to the 9. December I will stay on the AGU Meeting in San Francisco. You can visit me in the oral sessions or during coffee breaks.
I am looking forward to meeting you on the conference.
Presentation:
I will present a talk in the session: Advances in Signal Processing Methods for Seismology IV at 2:55 PM - 3:10 PM Room 2009 and the title is: "Horizontal Ocean-Bottom-Sensor sediment coupling; Estimation of coupling parameters from seismic data" (S33B-06).
Abstract:
The reliable replication of seismic waves depends on the interaction of the Ocean-Bottom-Cable (OBC) with the seabed, regardless of the direction in which the wave travels. The interaction is called coupling and is typically better on the in-line sensor-component because of the surface enhancing effect of the cable. Inconsistent coupling of multi-component sensor-nodes causes distortions between the horizontal components and this makes the interpretation of converted wave difficult. Horizontal OBC data are often characterized as “ringy” and have different noise levels between inline and crossline. We will show that these characteristics are expected if coupling to the sediment is poor. Coupling and data quality are generally good for the inline component, except for a higher noise floor caused by cable noise. However, the crossline component often exhibits low-frequency resonance. Also, OBCs are susceptible to rotational modes about the cable axis that produce spurious ‘S-waves’ resonance on the vertical component.
We will present a method to estimate the coupling parameters for both horizontal components independently by using a “feed-back transfer-function” method. The result can be used to optimize the sensor-housing design or to apply an inverse filter in order to extract the coupling transfer-function from the data. The presentation will show that inconsistent coupling of horizontal components can be estimated by using a data-driven approach. The presenting method estimates the two coupling parameter direct from the first arrival wave (first-break) without any affected earth-responses. Neither assumptions like perfect inline coupling have to be made nor will in-situ measurements such as internal shakers be necessary to estimate the coupling parameters.
EAGE Annual Conference & Exhibition 2011 Vienna
I will summarize the talks on the conference presentations and the short-course I have attended on the EAGE. The short-course was “An Introduction to Velocity Model Building” by Ian Jones and the focus was set to build a detailed velocity model. My foci on the technical sessions were Microseismic, seismic Interferometry. Beside those I attended some presentation about Full waveform inversion and time-lapse seismic, but nothing new was presented.
Short-course
I attened the short-course which took place at the 27.05.2011.
This short-course presented the basic principles of migration and of building a velocity model of the earth’s subsurface. The focus was set to the underlying principles and pitfalls. We discussed e.g. how is a velocity model typically built and what are the consequences of not getting it right.
The short-course concludes with a look at emerging and future trends: the promise of velocity-independent imaging and the potential of full waveform inversion. But this is still “basic” research…
Conference
On this conference was a huge interest in microseismic methods and most of the presentations pointed out how to get more information out of the data. In the last couple years microseismic detection was the key research branch, but now complex microseismic processing techniques starting to reach commercial value and becoming more and more reliable to get valid information out of the microseismic data.
I think that following methods are necessary to monitor hydraulic fractureing and cap-rock integrity with microseismic and some interesting talks were presented:
- Source mechanism and B-value estimation
- Full elastic wave inversion
- Microseismic monitoring design
- Magnitude estimation by fluid injections in reservoirs
- Seismic interferometry using ambient noise (passive method)
- Simultaneous-source method including Interferometry and Tomography
In the next several years I expect an increase of micro- and passive-seismic methods to become commercial and will be used in parallel to classical 3D/4D active seismic surveys.
Third Passive Seismic Workshop 2011 in Athens, Greece
Abstract
I will summarize the talks on the workshop. The foci on the technical sessions were detecting hydraulic fractures, focal mechanism, pre-processing methods and feasibility studies of microseismic monitoring.
Focal mechanism
Fluid injection will cause microseismic fractures in the “tight” sand lenses or reservoir. The wave-form inversion calculation of the focal mechanism by the moment tensor can estimate tensile openings and closures. This is an important step to understand what really happens in the reservoir or overburden. The result can “easily” be plotted into a Hudson-diagram and shows the opening and closing of fractures during injections over time.
To do a proper wave-form inversion it is necessary to have accurate velocity-models, estimated rock parameters (e.g. attenuation, resonance frequency, Q-factor…) and an understanding of the source mechanism. All this parameters are difficult to get, and assumptions and simulations should be done in a feasibility study.
Microseismic monitoring and imaging
From my point of view microseismic is now a potential tool to image hydraulic fracturing and to investigate it. Typically, hydraulic fracturing is considered as a single tensile fracture opening orthogonal to the minimum principal stress direction. But this is not every time the case. Microseismic imaging showed that there is a significant variability of fracturing, even over relatively short distances. Hydraulic fractures are not only simple fracture planes. They can also be complex fracture networks with different fracture directions and vary in height and length from meters to millimeters rapidly over short distances. This variability points to the value of microseismic monitoring and imaging. The results can be used to better design hydraulic fracture stimulation on the one hand and cap-rock protection on the other hand. The results can also be used to present the behavior in the reservoir or cap-rock to reservoir engineers to use as a prediction tool. The workshop focus was not entirely related to microseismic monitoring and imaging, but I think this will be the next necessary step to convince the oil and gas industry to assess probability of success (PoS) and value of information (VoI). The first step in a microseismic project should be a feasibility study to clearly articulate the purpose of the monitoring campaign. Beside assessing PoS and VoI the feasibility study should highlight the following steps:
- Modeling of the microseismic event detectability and related location errors.
- Estimate the magnitude of the smallest detectable event; considering the ambient noise-floor compared to the sensor noise-level.
- Design the sensor-array (surface and borehole) to improve detectability and to reduce location errors.
- Simulation of existing/known fractures to estimate focal mechanisms and stress regime.
- Full wave simulation to anticipate seismic features like multiples, surface waves etc. which are able to hinder event-time picking.
The common way to model detectability is to consider P and S arrivals from a double-couple source of a given moment magnitude for each sensor-location. The minimum detectable magnitude can be estimated of the comparison between the calculated arrival amplitude and the assumed sensor noise floor. A new approach is using the seismicity rate of microseismic events and the fluid injection rate to calculate the “seismogenic index”. This index can be used to quantitatively compare different seismic locations and the potential seismicity to estimate the magnitude.
Unfortunately, the minimum magnitude says nothing about the seismic activity. A systematic estimate of seismic activity is important to estimate the number of detectable events. McGarr proposed a model to estimate seismic failure related to hydraulic fracturing. The total seismic moment released during hydraulic fracturing is the product of injected volume and rock-shear modulus. The relationship between the number of events and magnitude is typically given by the Gutenberg-Richter scale. This McGarr model usability is still under discussion and can maybe not be used for aseismic deformation.
These results can guide to an improved sensor-array design with help of ray-tracing and full wave simulation.
Pre-Processing
The problem of seismic event detection is related to the localization methods which require picking of P- and S-waves arrival time. The accuracy of picking the correct arrival time is often strongly affected by all types of ambient noise.
Stacking is a common way to increase the signal to noise ratio (SNR), but the amplitude is often not satisfactory due to source mechanism and polarity changes. Another possibility is to stack the energy instead of the amplitude, but this is also not optimal due to missing differentiation between signal and noise. Correlation methods are maybe more reliable, but need a reference signal like a perforation shot response (after getting the correct shot-location and calibrated velocity model).
All pre-processing methods need additional information, assumptions or estimations and the accuracy of the event location or focal mechanism is related to a good model.
Assuming a good sensor coupling shear-wave splitting could be an option. When an S-wave travels through an anisotropic medium it will split into two orthogonally polarized waves and one will travel faster than the other. The polarization of the fast wave and the time between both S-wave arrivals defines the splitting along the travel-path. This can be used to eliminate surface waves or noise.
The most promising pre-processing method will be polarization and Eigen-value analysis in combination of the signal energy (referring to mala). Using the polarization will help to separate ambient noise (mostly not polarized) from microseismic signal (polarized).
Why does surface monitoring work?
There are still some discussions ongoing related to the question why surface monitoring works. The noise level in the borehole is usually lower than on the surface and the maximum detecting distance from borehole measurements are at best 1500 meter away from the sensor. Borehole measurements are using inversion techniques to locate and analyze microseismic events. The SNR has to be above one. On the other side surface arrays rely on imaging or pre-processing techniques, because the SNR is below one. It is also well known, that the source mechanism affects both borehole and surface array signals.
At the bottom-line of the discussion: we measure microseismic signals on the surface!
One possible explanation is that the near-surface layers increase the signal amplitude of microseismic events due to impedance contrast changes and free surface boundary conditions. This effect can compensate the increased noise floor on the surface and can reduce the numbers of surface sensors.
Another explanation is related to the fact that the earth’s stress regime is only increasing due to gravity and density. A down-going signal will lose energy because of attenuation, but on the way up it will regain energy due to the velocity layers and the stress regime.
SEG Exhibition & Annual Meeting 2010 in Denver
I will present a talk in the Session ACQ 2 -- Survey Design and Marine OBS,Wednesday, October 20, 2010; Room: 103/105.
INOSACS
Investigation and optimization of OBC sensor array coupling to the seafloor
The most important challenge for the oil-industry is to increase the recovery rates for existing fields and to map fluid movements with time-laps 4D seismic, reducing geo-hazard like cap-rock integrity, subsidence or to monitor CO2 storage in an offshore reservoir by using active and passive sources (e.g. Airgun-survey and passive/microseismic). The seismic equipment is configured as sensor lines with cables trenched and covered on the sea-bottom.
The equipment typically comprises up to 4000 sensor nodes depending on the aerial extent of the reservoir. The system will preferably be connected directly to onshore operation centers by means of broadband communication and data can be controlled and processed for QA purposes in real time. Traditional 4D seismic techniques re-shoot a specific reservoir using conventional towed streamer techniques, trying to map and assess changes in the reservoir by having a repeat interval of 2-3 years.
I started in September 2010 a new research project in cooperation with Octio Geophysical AS and the University of Bergen.
There will be two main objectives for this project. The first will be a systematic investigation of OBC coupling to the seafloor for a better understanding of the horizontal coupling mechanism to increase the signal quality for 4D seismic, micro/passive seismic and fluid flow in a reservoir by injection of water or super critical CO2.
The second objective is to design a new OBC sensor-housing (node) to improve the coupling and a test on a real reservoir in cooperation with a costumer or in an addition research project.
In order to improve sensor coupling in the offshore environment novel and systematic approaches are needed. Today several approaches exist for he vertical component of a three component sensor. But there are no systematic investigations for the horizontal components. Most of the vertical approaches use special pre-processing methods like separation of upgoing and downgoing P and S wave-fields or inverse filtering.
But to understand the sensor coupling to the seafloor it is essential to investigate the sensor coupling itself. or a systematic investigation it is substantial to understand the connection f different sensor-housing designs to variable seafloors. Which design makes the sensor coupling better and why? What influence has the seafloor oil conditions and how deep should an OBC be trenched and how strong is he cable-tension influence? Why is the sensor coupling different for all three components?
All these questions will be investigated by using simulations and doing tests in a water-tank. The water tank will be hanged to the floor to decouple the water-tank from the ambient noise.
The most critical challenges will be to simulate the correct soil coupling ith the sensor-housing for horizontal movements, because current models its not into measurements and can only be sed as a start model. To find the optimal mechanical node design is also a critical challenge, because of to integrate the new design into the offshore installation procedures.
